1. Field of the Invention
The present invention relates in general to a fluid-filled resilient or elastic bushing structure, and more particularly to such a bushing structure which is capable of exhibiting excellent spring characteristics for both low-frequency vibrations and high-frequency vibrations that are applied thereto in a diametric direction of the structure.
2. Discussion of the Prior Art
There is known a resilient bushing for elastically connecting two members in a vibration system (through which vibrations are transmitted), for damping and/or isolating vibrations applied to the bushing in a given diametric direction of the bushing. The bushing has an inner sleeve in which a mounting rod or bolt is inserted, an outer sleeve on which a cylindrical mounting member is fitted, and a resilient member interposed between the inner and outer sleeves. For example, such a resilient bushing is used as suspension bushings in a vehicle suspension system, or as engine mounts for mounting a power unit on the body of an F-F vehicle (front-engine, front drive vehicle).
Usually, the resilient bushing of the type indicated above is required to exhibit high vibration isolating characteristic for high-frequency vibrations having a small amplitude, and high vibration damping characteristic for low-frequency vibrations having a large amplitude. The traditional resilient bushing relies solely on the elastic nature (elastic deformation) of a resilient or elastic member, to provide both the vibration isolating capability and the vibration damping capability. Therefore, the bushing is difficult to satisfy these two different requirements. In particular, the traditional resilient bushing is not satisfactory in its capability of damping the low-frequency vibrations of large amplitudes.
In the light of the above inconvenience, a fluid-filled resilient bushing has been proposed in recent years. An example of such a fluid-filled bushing is disclosed in U.S. Pat. Nos. 3,642,268 and 3,698,703. This fluid-filled bushing has a pair of fluid chambers formed in an annular resilient member such that the fluid chambers are located opposite to each other in a diametric direction of the bushing in which vibrations are applied. These fluid chambers are filled with a suitable incompressible fluid, and communicate with each other through an orifice, so that the fluid may flow through the orifice, between the two chambers, upon application of low-frequency vibrations of a large amplitude in the diametric direction.
In the fluid-filled bushing indicated above, the input low-frequency vibrations can be effectively damped due to inertia and resonance of the fluid mass in the orifice when the fluid is forced to flow through the orifice. The frequency range of the vibrations to be damped can be selected by suitably dimensioning the orifice.
If the orifice of this type of fluid-filled resilient bushing is dimensioned (in terms of its length and cross sectional area or diameter) so as to provide excellent damping characteristic for vibrations in a low frequency range, then the vibration isolating capability of the bushing is accordingly reduced for the high-frequency vibrations having a small amplitude. Thus, there has been a need to develop a fluid-filled resilient bushing which is satisfactory in the overall vibration damping and isolating capability or characteristic.